corrected sentence:
The store is offering "both" books which are filled with coupons and free T-shirts to shoppers who purchase more than $100 worth of products.
The sentence has no grammar mistakes,but there is a logical problem.
用中文说就是你的句子里,books which are filled with coupons and free T-shirts这句意思不清楚
可以理解为books里装满了coupons and free T-shirts
但是实际上是books which are filled with coupons 和 free T-shirts是独立的两种物品,用both的话就可以区别and前和and后的物品了